Lepraria diffusa was originally described by Jack Laundon in 1989 as Leproloma diffusum, based on a type specimen collected in Finland.

He determined that the colour differences resulted from varying levels of 4-oxypannaric acid 2-methylester, influenced by environmental factors like sunlight.

Chemical analyses showed no distinct taxonomic traits, and intermediate specimens suggested continuous variation, leading to the recognition of a single taxon.

[5] Lepraria diffusa is a leprose lichen forming a powdery to cottony thallus.

The thallus contains abundant coarse soredia measuring up to 100 micrometres in diameter, with projecting hyphae sometimes present and short in length.